African swine fever (ASF), caused by the ASF virus (ASFV), is an acute, severe, and highly contagious infectious disease in domestic pigs and wild boars. Domestic pigs infected with a virulent ASFV ...strain can have morbidity and mortality rates of up to 100%. The epidemic of ASF has caused serious economic losses to the global pig industry. Currently, there is no safe and effective vaccine or specific drug for treating ASF. Therefore, ASFV still poses a great threat to pig factories. ASFV is a double-stranded DNA virus with a complex icosahedral multilayer structure. The ASFV genome contains 150–170 open reading frames (ORFs) that encode 150–200 proteins. Some ASFV-encoded proteins are involved in virus invasion, genome replication, DNA repair, and virion formation. Some ASFV proteins execute immunomodulatory functions by regulating the host antiviral innate immune response. Accumulating studies have shown that the immunomodulatory functions of ASFV genes are closely related to the virulence and pathogenicity of ASFV isolates. This review summarizes the research advances on ASFV immune evasion mechanisms in African swine fever patients and provides new insights for developing attenuated live vaccine candidates to prevent and control ASF.
Inflammatory factors and type I interferons (IFNs) are key components of host antiviral innate immune responses, which can be released from the pathogen-infected macrophages. African swine fever ...virus (ASFV) has developed various strategies to evade host antiviral innate immune responses, including alteration of inflammatory responses and IFNs production. However, the molecular mechanism underlying inhibition of inflammatory responses and IFNs production by ASFV-encoded proteins has not been fully understood. Here we report that ASFV infection only induced low levels of IL-1β and type I IFNs in porcine alveolar macrophages (PAMs), even in the presence of strong inducers such as LPS and poly(dA:dT). Through further exploration, we found that several members of the multigene family 360 (MGF360) and MGF505 strongly inhibited IL-1β maturation and IFN-β promoter activation. Among them, pMGF505-7R had the strongest inhibitory effect. To verify the function of pMGF505-7R
in vivo
, a recombinant ASFV with deletion of the MGF505-7R gene (ASFV-Δ7R) was constructed and assessed. As we expected, ASFV-Δ7R infection induced higher levels of IL-1β and IFN-β compared with its parental ASFV HLJ/18 strain. ASFV infection-induced IL-1β production was then found to be dependent on TLRs/NF-κB signaling pathway and NLRP3 inflammasome. Furthermore, we demonstrated that pMGF505-7R interacted with IKKα in the IKK complex to inhibit NF-κB activation and bound to NLRP3 to inhibit inflammasome formation, leading to decreased IL-1β production. Moreover, we found that pMGF505-7R interacted with and inhibited the nuclear translocation of IRF3 to block type I IFN production. Importantly, the virulence of ASFV-Δ7R is reduced in piglets compared with its parental ASFV HLJ/18 strain, which may due to induction of higher IL-1β and type I IFN production
in vivo
. Our findings provide a new clue to understand the functions of ASFV-encoded pMGF505-7R and its role in viral infection-induced pathogenesis, which might help design antiviral agents or live attenuated vaccines to control ASF.
The NLRP3 inflammasome plays a major role in innate immune responses by activating caspase-1, resulting in secretion of IL-1β and inflammatory pathologic responses. Viral RNA can induce NLRP3 ...inflammasome activation. However, none of the components of NLRP3 inflammasome has the ability to bind viral RNA. Therefore, it had been proposed that there might have been some unidentified cytosolic RNA sensors that could bind viral RNA and NLRP3 to initiate NLRP3 inflammasome activation. In this study, DDX19A, a member of the DEAD/H-box protein family, was identified as a novel component of NLRP3 inflammasome using arterivirus infection as a model. We found that DDX19A interacted with viral RNA and NLRP3. Knockdown of DDX19A expression efficiently inhibited procaspase-1 cleavage and IL-1β secretion in porcine reproductive and respiration syndrome virus (PRRSV)-infected or PRRSV RNA-stimulated primary porcine alveolar macrophages. Overall, DDX19A was identified as a novel cytosolic RNA sensor that bridged PRRSV RNA and NLRP3 to activate NLRP3 inflammasome.
The non-specific innate immunity can initiate host antiviral innate immune responses within minutes to hours after the invasion of pathogenic microorganisms. Therefore, the natural immune response is ...the first line of defense for the host to resist the invaders, including viruses, bacteria, fungi. Host pattern recognition receptors (PRRs) in the infected cells or bystander cells recognize pathogen-associated molecular patterns (PAMPs) of invading pathogens and initiate a series of signal cascades, resulting in the expression of type I interferons (IFN-I) and inflammatory cytokines to antagonize the infection of microorganisms. In contrast, the invading pathogens take a variety of mechanisms to inhibit the induction of IFN-I production from avoiding being cleared. Pseudorabies virus (PRV) belongs to the family Herpesviridae, subfamily Alphaherpesvirinae, genus Varicellovirus. PRV is the causative agent of Aujeszky's disease (AD, pseudorabies). Although the natural host of PRV is swine, it can infect a wide variety of mammals, such as cattle, sheep, cats, and dogs. The disease is usually fatal to these hosts. PRV mainly infects the peripheral nervous system (PNS) in swine. For other species, PRV mainly invades the PNS first and then progresses to the central nervous system (CNS), which leads to acute death of the host with serious clinical and neurological symptoms. In recent years, new PRV variant strains have appeared in some areas, and sporadic cases of PRV infection in humans have also been reported, suggesting that PRV is still an important emerging and re-emerging infectious disease. This review summarizes the strategies of PRV evading host innate immunity and new targets for inhibition of PRV replication, which will provide more information for the development of effective inactivated vaccines and drugs for PRV.
African swine fever virus (ASFV) is the etiological agent of African swine fever (ASF), and it is an enveloped, icosahedral, double-stranded DNA virus with a genome length ranging from 170 to 193 kb. ...The ASFV genome contains at least five multigene families (MGFs): MGF100, MGF110, MGF300, MGF360, and MGF505 at the right and left terminal variable regions. The members of the same MGF family are most similar and have conserved sequence motifs, whereas the genetic diversity of different MGF families varies widely. MGF genes play a crucial role in determining ASFV host range, virulence and reducing early cell death post-infection. pMGF505-7R is a multifunctional protein. Recent research advances of pMGF505-7R provide a few new clues to understand the functions of pMGF505-7R either in antagonizing the induction of type I IFN production and IFN downstream signaling or in suppressing inflammatory responses by inhibition of NF-κB signaling and NLRP3 inflammasome, which may be related to ASFV infection-induced pathogenesis.
Apoptosis is a critical host antiviral defense mechanism. But many viruses have evolved multiple strategies to manipulate apoptosis and escape host antiviral immune responses. Herpesvirus infection ...regulated apoptosis; however, the underlying molecular mechanisms have not yet been fully elucidated. Hence, the present study aimed to study the relationship between herpesvirus infection and apoptosis in vitro and in vivo using the pseudorabies virus (PRV) as the model virus. We found that mitochondria-dependent apoptosis was induced by PRV gM, a late protein encoded by PRV UL10, a virulence-related gene involved in enhancing PRV pathogenicity. Mechanistically, gM competitively combines with BCL-XL to disrupt the BCL-XL-BAK complex, resulting in BCL-2-antagonistic killer (BAK) oligomerization and BCL-2-associated X (BAX) activation, which destroys the mitochondrial membrane potential and activates caspase-3/7 to trigger apoptosis. Interestingly, similar apoptotic mechanisms were observed in other herpesviruses (Herpes Simplex Virus-1 HSV-1, human cytomegalovirus HCMV, Equine herpesvirus-1 EHV-1, and varicella-zoster virus VZV) driven by PRV gM homologs. Compared with their parental viruses, the pathogenicity of PRV-ΔUL10 or HSV-1-ΔUL10 in mice was reduced with lower apoptosis and viral replication, illustrating that UL10 is a key virulence-related gene in PRV and HSV-1. Consistently, caspase-3 deletion also diminished the replication and pathogenicity of PRV and HSV-1 in vitro and in mice, suggesting that caspase-3-mediated apoptosis is closely related to the replication and pathogenicity of PRV and HSV-1. Overall, our findings firstly reveal the mechanism by which PRV gM and its homologs in several herpesviruses regulate apoptosis to enhance the viral replication and pathogenicity, and the relationship between gM-mediated apoptosis and herpesvirus pathogenicity suggests a promising approach for developing attenuated live vaccines and therapy for herpesvirus-related diseases.
Stem cell self-renewal is controlled by concerted actions of extrinsic niche signals and intrinsic factors in a variety of systems. Drosophila ovarian germline stem cells (GSCs) have been one of the ...most productive systems for identifying the factors controlling self-renewal. The differentiation factor BAM is necessary and sufficient for GSC differentiation, but it still remains expressed in GSCs at low levels. However, it is unclear how its function is repressed in GSCs to maintain self-renewal. Here, we report the identification of the translation initiation factor eIF4A for its essential role in self-renewal by directly inactivating BAM function. eIF4A can physically interact with BAM in Drosophila S2 cells and yeast cells. eIF4A exhibits dosage-specific interactions with bam in the regulation of GSC differentiation. It is required intrinsically for controlling GSC self-renewal and proliferation but not survival. In addition, it is required for maintaining E-cadherin expression but not BMP signaling activity. Furthermore, BAM and BGCN together repress translation of E-cadherin through its 3' UTR in S2 cells. Therefore, we propose that BAM functions as a translation repressor by interfering with translation initiation and eIF4A maintains self-renewal by inhibiting BAM function and promoting E-cadherin expression.
Pseudorabies, caused by the pseudorabies virus (PRV), is an acute fatal disease, which can infect rodents, mammals, and other livestock and wild animals across species. Recently, the emergence of PRV ...virulent isolates indicates a high risk of a variant PRV epidemic and the need for continuous surveillance. In this study, PRV-GD and PRV-JM, two fatal PRV variants, were isolated and their pathogenicity as well as their effects on host natural immune responses were assessed. PRV-GD and PRV-JM were genetically closest to PRV variants currently circulating in Heilongjiang (HLJ8) and Jiangxi (JX/CH/2016), which belong to genotype 2.2. Consistently, antisera from sows immunized with PRV-Ea classical vaccination showed much lower neutralization ability to PRV-GD and PRV-JM. However, the antisera from the pigs infected with PRV-JM had an extremely higher neutralization ability to PRV-TJ (as a positive control), PRV-GD and PRV-JM. In vivo, PRV-GD and PRV-JM infections caused 100% death in mice and piglets and induced extensive tissue damage, cell death, and inflammatory cytokine release. Our analysis of the emergence of PRV variants indicate that pigs immunized with the classical PRV vaccine are incapable of providing sufficient protection against these PRV isolates, and there is a risk of continuous evolution and virulence enhancement. Efforts are still needed to conduct epidemiological monitoring for the PRV and to develop novel vaccines against this emerging and reemerging infectious disease.
DExD/H-box helicase members are key receptors for recognizing viral nucleic acids, and they regulate retinoic acid-inducible gene I (RIG-I)-like receptor (RLR)-mediated type I interferon (IFN) ...production. Here, we report that the DExD/H-box helicase family member DExD/H-box RNA helicase 19 (DDX19) is a negative regulator of type I IFN production. Ectopic expression of DDX19 suppressed poly(I:C) (polyinosinic-polycytidylic acid)- and Sendai-virus-induced type I IFN production, whereas knockdown of DDX19 expression enhanced type I IFN production. Mechanistically, DDX19 inhibited TANK-binds kinase 1 (TBK1)- and inhibitor-κb kinase ε (IKKε)-mediated phosphorylation of interferon regulatory factor 3 (IRF3) by disrupting the interaction between TBK1 or IKKε and IRF3. Additionally, DDX19 recruited Lamtor2 and then formed the TBK1-IKKε-Lamtor2-DDX19-IRF3 complex to suppress IFN production by promoting TBK1 and IKKε degradation. We generated Ddx19 knockout mice using transcription activator-like effector nucleases (TALENs) and found that Ddx19 deficiency in vivo augmented type I IFN production, resulting in suppression of encephalomyocarditis virus replication. These data show that DDX19 is an important negative regulator of RLR-mediated type I IFN production.
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•DDX19 suppresses IRF3 phosphorylation•DDX19 competes with TBK1 or IKKε binding to the IAD domain of IRF3•DDX19 recruits Lamtor2 to promote TBK1 and IKKε degradation•DDX19 inhibits type I IFN production and thus enhances viral replication
DExD/H-box helicase members are key receptors for recognizing viral nucleic acids and participate in regulating RLR-mediated type I IFN production. Zhang et al. show that DDX19 acts as a critical negative regulator of type I IFN production by disrupting TBK1-IKKε-IRF3 complex formation and recruiting Lamtor2 to degrade TBK1 and IKKε.
African swine fever (ASF) caused by African swine fever virus (ASFV) is a highly mortal and hemorrhagic infectious disease in pigs. Previous studies have indicated that ASFV modulates interferon ...(IFN) production. In this study, we demonstrated that ASFV pA151R negatively regulated type I IFN production. Ectopic expression of pA151R dramatically inhibited K63-linked polyubiquitination and Ser172 phosphorylation of TANK-binding kinase 1 (TBK1). Mechanically, we demonstrated that E3 ligase TNF receptor-associated factor 6 (TRAF6) participated in the ubiquitination of TBK1 in cGAS-STING signaling pathway. We showed that pA151R interacted with TRAF6 and degraded it through apoptosis pathway, leading to the disruption of TBK1 and TRAF6 interaction. Moreover, we clarified that the amino acids H102, C109, C132, and C135 in pA151R were crucial for pA151R to inhibit type I interferon production. In addition, we verified that overexpression of pA151R facilitated DNA virus Herpes simplex virus 1 (HSV-1) replication by inhibiting IFN-β production. Importantly, knockdown of pA151R inhibited ASFV replication and enhanced IFN-β production in porcine alveolar macrophages (PAMs). Our findings will help understand how ASFV escapes host antiviral immune responses and develop effective ASFV vaccines.
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